In the field of oil and gas production, a reciprocating beam pumping unit or “pumpjack” (referred to herein simply as a “pumping unit”) is a form of counterbalanced reciprocating device used to drive a downhole pump to lift reservoir fluids in wells with insufficient bottom-hole pressure to lift reservoir fluids to the surface at a desired production rate. The pumping unit is an above ground drive unit that includes a rotating motor to drive a crankshaft (typically through a reducing gear arrangement) and a suitable linkage arrangement between the crankshaft and a walking beam. In operation, the walking beam is driven to pivot back and forth about a suitable pivot structure to provide an effective mechanical reciprocating motion to drive the downhole reciprocating pump. The downhole pump can be within the well hundreds or even thousands of feet below the surface. The connection to the above ground pumping unit is through a series of elongated interconnected rods known as sucker rods which extend typically through a string of production tubing from the surface to the location of the downhole pump. The reciprocating action of the above ground pumping unit raises and lowers the entire length of the sucker rods to drive the downhole pump to lift reservoir fluids which have entered the well bore.
On each upstroke of the pumping unit, the pumping unit must lift not only the entire length of sucker rods and the reciprocating portion of the downhole pump, but also the entire column of reservoir fluids in the production tubing. The weight lifted with each stroke can sometimes exceed twenty thousand pounds. As a practical matter, lifting such weight requires a counterweight arrangement on the pumping unit. One type of counterweight arrangement includes counterweights located on the walking beam, opposite the pivot point from the downhole weight. These “beam-balanced” pumping units suffer from reduced stroke length and reduced pump capacity. Also, they become relatively unbalanced at larger angles of walking beam tilt. Therefore, they have been limited to use in relatively shallow wells. Another type of counterweight arrangement, which can be used with or without counterweights located on the walking beam, includes counterweights located on the rotating crankshaft of the driving reduction gear. In these “crank-balanced” pumping units the rotation of the counterweights mounted on the crankshaft has a horizontal force vector in all but the twelve and six o'clock positions. Reducing this to horizontal and vertical vectors only, fifty percent (50%) of the work used to move the counterweights is horizontal, and thus ineffective. Because the drive unit directly effects counterweight movement and indirectly moves the rod-pump complex through the walking beam, only the vertical forces on the counterweights produce vertical movement on the downhole pump components. Thus fifty percent (50%) of the power drive unit work expended in a crank-balanced pumping unit can be ineffective in pump output. Frequency driving the drive motor for the pumping unit differentially in different portions of the rotational cycle can reduce power consumption during less vertically efficient portions of the rotational stroke cycle. However, because the vertical and horizontal components cannot be completely isolated, the reduction of ineffective power consumption is limited.
Even with prior art counterweight arrangements, driving the pumping unit requires considerable energy input from the motor, which is commonly an electric motor. Because the upward stroke of the pumping unit and downhole reciprocating pump produce a relatively low volume of pumped fluids, between 5-40 liters per stroke, long run-times for these pumps can consume relatively large amounts of energy. This energy consumption is part of the calculated “lift cost,” which reflects the relative efficiency and profitability of such production wells.
Attempts to improve artificial lift systems which utilize a pumping unit have included improvements in materials of construction, design improvements in critical components such as bearing surfaces, reduction gearing, variations in counter-weight balancing, stroke mechanics and overall harmonics of rotary and reciprocal actions, and the use of frequency drive systems to more efficiently match mechanical harmonics to motor drive output throughout the pump cycle. Also, reducing downhole weights which must be lifted with each pumping cycle by use of lighter weight components can reduce pumping work directly. The benefits of lighter weight downhole components sometimes are off-set by increased failure rates and reduced capacity.